using System;

public class Program

{

    public static void Main()

    {

        int k = 10;

        int count = 0;

        int[] arr = new int[]{3,5,2,1,-3,7,8,15,6,13};

        for(int i = 0;i<arr.Length;i++)

        {

            for(int j = i+1;j<arr.Length;j++)

            {

                if(arr[i]+arr[j] == k)

                count++;

            }

        }

        Console.WriteLine("Count of pairs whose sum is k :"+count);

    }

}